Dependence of pulmonary venous admixture on inspired oxygen fraction and time during regional hypoxia in the rabbit.

In order to examine the value of assuming constant pulmonary venous admixture with respect to changes in inspired oxygen fraction (FIO2) and time during sustained unilateral hypoxia, we studied venous admixture for 6 h in 27 anaesthetized rabbits in which the left lung was filled with liquid, isosmotic with plasma. In one group of 10 rabbits the right lung was ventilated for 6 h with FIO2 = 1; in a second group of 10 the right lung was ventilated with FIO2 = 1 for 2.5 h and then with FIO2 = 0.3 for 3.5 h. A third group was similarly studied by changing from FIO2 = 1 to FIO2 = 0.5. We found that hypoxic pulmonary vasoconstriction continued to intensify over 3 h. At 3-6 h, with FIO2 = 0.3, venous admixture (0.32 (SEM 0.03)) was higher than baseline (0.13 (0.01), t = 0 min during bilateral oxygenation) by twice the elevation above baseline of the venous admixture (0.22 (0.01)) in the group with FIO2 = 1. The finding of a marked increase in venous admixture with decreasing FIO2 is discussed in relation to current models of hypoxic pulmonary vasoconstriction.